Problem: The scale on a map is 6cm : 3km. If the distance between two cities is 24km, how far apart in cm are the two cities on the map?
The scale means that every 6cm on the map corresponds to an actual distance of 3km. An actual distance of 24km is the same as 8 $\cdot$ 3km. The distance between the two cities on the map is 8 $\cdot$ 6cm, or 48cm.